How can growers prepare before a freeze event
Temperatures are forecasted to drop below freezing point Friday night. Minimum temperatures will reach 27-26 °F in Southern Wisconsin were apple growers are reporting tight cluster to pink stages in early apple cultivars such as Zestar, Pink Lady, Ginger Golden, Paulas, and Idared. In other locations in the state, trees are not so advance in their phenological stage and this will impact the resistance to freezing temperature. As cluster begin to open, they become less resistant to freeze injury. The table below shows the average temperatures required to kill 10 percent and 90 percent of buds if they are exposed for 30 minutes.
| Stage of Development | 10% kill (°F) | 90% kill (°F) |
| Silver tip | 15 | 2 |
| Green tip | 18 | 10 |
| ½-inch green | 23 | 15 |
| Tight cluster | 27 | 21 |
| First pink | 28 | 24 |
| Full pink | 28 | 25 |
| First bloom | 28 | 25 |
| Full bloom | 28 | 25 |
| Post bloom | 28 | 25 |
What can growers do to prepare for a possible freeze?
1) Improve air flow/drainage in the orchard will reduce the likelihood of freeze injury during the freeze event, make sure to remove impediments to the natural air drainage path in the orchard.
2) Irrigation and weed control can also help reduce freeze damage. Soil that is moist, weed-free, and undisturbed will capture and retain more heat than dried, freshly cultivated, or weed-covered soil.
3) Mowing alley ways as close to the ground as possible. This will allow for more heat to be capture during the day to be released at night.
4) Check and test wind machines/frost fans ahead of time.
5) Check your irrigation system and keep your soil moist, it will capture more heat during the next 3 sunny days we will have prior to Friday night, and radiate the heat back out the night of the freeze. You can also irrigate (trickle) at night during the freeze.
6) Promalin sprays. The recommended timing to apply Promalin is at pink stage to petal fall within 24-48 hours after the freeze event. If you are not yet at pink stage, I would recommend growers do not apply Promalin. Promalin will not be effective if applied at the green tip stage, 1/4-inch stage, or 1/2-inch stage, and it will not help a damaged bud to continue its development the following weeks either. Promalin can only improve fruit set when flowers (not buds) are damaged but not totally killed by a frost. The recommended rate of application is 1pint (16 oz) of Promalin per acre using 100-150 gallons of water per acre within 24-48 hours of the frost event. Grower’s will need to check the level of damage in their orchards before making a decision regarding applying Promalin.
How to assess the extend of the damage after the freeze event?
After the freeze event you should collect blossoms to assess the extend of the damage. It will take between 12 to 24 hours for the damage (browning of flowers structure) to express, so if you go sample flowers the morning after the freeze you will not see much, you have to wait 12 to 24 hrs. Choose 15 blossom clusters on 5 trees per block, select the cluster from the top, middle, and bottom of the canopy. Since the king blossom will be the most advanced flower in the cluster, it is most likely to be killed in a frost, so you should check the king and side blooms separately. The pistil of the flower in apples is located at the base of the flower, which means that is necessary to tear the flower apart to see if the center of the flower is brown or black (Figure 1).
